The present invention relates to a light guide plate which is supplied with light sideways and deflects the light to output from an emission face, further relating to a surface light source device employing the light guide plate as well as to a liquid crystal display employing the surface light source device for back-lighting or front-lighting.
A surface light source device of a type comprises a light guide plate having an end face, through which light is introduced, and two major faces (i.e. faces larger than end faces) one of which provides an emission face, being employed for various uses such as back-lighting or front-lighting for a liquid crystal display. Basic performance of surface light source devices of such type greatly depends on light guide plates employed therein.
A basic function of a light guide plate is to change propagation direction (roughly in parallel with an emission face of the light guide plate) of light introduced into the light guide plate through a side end face so that the light is emitted through the emission face. As known well, a simply transparent light guide plate without any modification is capable of deflecting light slightly, providing unsatisfactory brightness. Consequently, any means for promoting emission through the emission face is required.
Means for promoting emission from a light guide plate relies upon one of the followings or some of them as combined.
(1) Scattering power within the light guide plate (light scattering guide plate);
(2) Emission face (a major face) provided with light diffusibility;
(3) Back face provided with light diffusibility;
(4) Emission face provided with light-refractive unevenness;
(5) Back face provided with light-refractive unevenness.
Ways based on (1) provide uniform and highly effective emission with ease. However, the emission is subject to have a preferential direction much inclined with respect to a frontal direction. (Usually, inclination is about 60 to 75 degrees to a normal with respect to the emission face.) Accordingly, an element (prism sheet) for modifying the inclined direction to the frontal direction must be arranged. Although employment of a light diffusion sheet brings some increase in frontal emission, it involves a wide light diffusion which leads to reduction in light energy efficiency.
Ways based on (2) or (3) hardly provide uniform and effective emission. The emission is also preferentially directed to oblique directions as in the case of (1). Increase of light diffusibility checks the efficiency because of factors such as wide range scattering or absorption by light scattering elements (e.g. white ink).
Ways based on (4) are capable of causing light to escape from the emission face with ease while positive direction conversions are less effected. Accordingly, emission with high efficiency is expected little. In particular, it is not advantageous that they fail to generate light which travels from the back face to the emission face.
Ways based on (5) positively generate light which travels from a back face to an emission face of a light guide plate, being free from wide range light scattering. Accordingly, the ways are latently capable of effectively generating emission directed to approximately frontal directions. However, in practice, prior arts fail to control propagating direction of emission sufficiently.
FIG. 1a to FIG. 1c illustrate examples employing the above (5). Referring to the figure, reference number 1 indicates a light guide plate made of a transparent material such as acrylic resin, the plate having a side end face to provide an incidence face 2. A primary light source L is disposed beside the incidence face 2 to be supplied with light from the primary light source L. One of two major faces 3, 4 of the light guide plate 1 provides an emission face 3. The other major face (called xe2x80x9cback facexe2x80x9d) is provided with a great number of recesses 5 with slopes 5a, 5b in profile.
Light emitted from the primary light source L is introduced into the light guide plate 1 through the incidence face 2. Upon encountering a recess, propagation light within the light guide plate 1 (as represented by G1, G2) is inner-reflected by one slopes 5a to be directed to the emission face 3. Inner-incidence angle is denoted by xcex8 and emission derived from beams G1, G2 is denoted by G1xe2x80x2, G2xe2x80x2. In other words, the slope 5a, which is relatively near to the incidence face 2 (or primary light source L) compared with the other slope 5b, provides an inner-reflection slope for direction conversion. This effect is sometimes called edge-lighting effect.
The recesses 5 are formed like dots or linear channels. As shown in FIG. 1a to FIG. 1c, formation pitch d, depth h or slope inclination xcfx86 of the recesses 5 is varied depending on distance from the incidence face 2. Such variations prevent brightness on the emission face 3 from varying depending on distance from the incidence face 2.
However, prior arts as shown in FIG. 1a to FIG. 1c are subject to the following problems.
1. Light is hard to reach a region behind the slope 5b as viewed from the incidence face 2. Therefore, reduction of formation pitch d hardly rises direction conversion efficiency and the emission face 3 is apt to show unevenness in brightness.
2. Sufficient direction control in a plane parallel to the incidence face 2 is not effected. For instance, if beams G1, G2 are parallel to the emission face 3 but not perpendicular to the incidence face 2, emission beams G1xe2x80x2, G2xe2x80x2 will be diverged to the right or left as viewed from the incidence face 2. Actually, there is considerable light component propagating not perpendicularly with respect to the incidence face 2 within the light guide plate 1. Accordingly, it is difficult to direct emission to a desirable angle or within a desirable angle range spatially (i.e. in both planes parallel and vertical to the incidence face 2).
3. Light is apt to leak through the back face 4 because direction conversion for generating light directed to the emission face 3 relies upon once-reflection (at slope 5a). That is, condition for total reflection is broken with ease at reflection for direction conversion. For instance, if beams G1xe2x80x2, G2xe2x80x2 are required to be directed to approximately frontal directions, inner-incidence angle xcex8 is set at about 45 degrees. This is roughly the same as the critical angle for interface between air and acrylic resin which is a typical material. Therefore, a considerable part of light propagating slightly downward leaks through the slope 5a. 
The present invention aims to solve the above problems of prior arts. That is, an object of the present invention is to improve a light guide plate, which introduces light through a side end face (incidence face) to emit light through an emission face, so that the plate has no region which light is hard to reach and emission direction is controllable both in plane parallel and vertical to an incidence face while light leaking through a back face scarcely occurs.
Another object of the present invention is to provide, by using the improved light guide plate, a surface light source device capable of effectively generating illumination light emission direction of which is controlled both in plane parallel and vertical to an incidence face with no direction modifying element such as prism sheet being employed.
Still another object of the present invention is to provide, by applying the above surface light source device to a back-lighting or front-lighting arrangement, a liquid crystal display which is easy to view from a desired direction.
The present invention achieves the above objects under a basic idea that direction conversion is effected by twice-inner-reflection at inner surfaces of a projection.
First, the present invention improves a light guide plate comprising two major faces to provide an emission face and a back face, and a side end face to introduce light.
According to a feature of the present invention, the back face of the light guide plate is provided with a great number of projections for conversion of light propagation direction and each of the projections includes a guide portion and a conversion output portion having a ridge on both sides of which a first reflection face and a second reflection face are formed.
The ridge, and the first and second reflection faces configurate a valley within each projection, the valley getting narrower and shallower according to distance from the guide portion.
This causes inner input light reaching the valley to be reflected by one of said first and second reflection faces and then reflected by the other reflection face so that inner output light directed to the emission face can be generated.
The ridges of the conversion output portions of the projections may run in directions distributed depending on positions on the back face. In this case, distribution of emission directions from the emission face can be controlled depending on distribution of running directions of the ridges.
The present invention provides an improved surface light source device employing the above light guide plate. The present invention improves a surface light source device comprising at least one primary light source and a light guide plate having two major faces to provide an emission face and a back face and having a side end face to introduce light emitted from the primary light source.
Corresponding to the above feature of the light guide plate, the back face is provided with a great number of projections for conversion of light propagation direction. Each of the projections includes a guide portion and a conversion output portion including a first ridge on both sides of which a first reflection face and a second reflection face are formed.
The first ridge, and the first and second reflection faces configurate a first valley within each of the projections. The first valley is getting narrower and shallower according to distance from the guide portion.
This causes inner input light reaching the first valley to be reflected by one of the first and second reflection faces and then reflected by the other reflection face so that inner output light directed to the emission face can be generated.
Running direction of the first ridge may vary depending on position on the back face. In this case, distribution of emission directions from the emission face can be controlled depending on distribution of running directions of the first ridges.
According to an embodiment, inner output light produced by the numerous projections is emitted from the emission face so as to be directed roughly parallel to a normal with respect to the emission face. And according to another embodiment, inner output light produced by the numerous projections is emitted from the emission face so as to form a convergent flux.
Light introduction may be done from a first direction and a second direction opposite to the first direction. In this case, the guide portion may have a valley-like configuration the same as or similar to that of the conversion output portion, with the conversion output portion also functioning like the guide portion.
To achieve this, the guide portion is also configurated like a valley so that light guiding function and conversion outputting function are exchanged with each other for the light introduced from the second direction, while the light introduced from the first direction gives inner input light which reaches the first valley via the guide portion to be reflected by one of the first and second reflection faces and then by the other reflection face so that inner output light directed to the emission face can be generated.
That is, the guide portion includes a second ridge on both sides of which a third reflection face and a fourth reflection face are formed, while the second ridge, the third reflection face and the fourth reflection face configurate a second valley within each of the projections, the second valley getting narrower and shallower according to distance from the conversion output portion.
This causes inner input light reaching the second valley via the conversion output portion after being introduced from the first direction to be reflected by one of the third and fourth reflection faces and then by the other reflection face so that inner output light directed to the emission face can be generated. The first and second valleys are preferably approximately the same dimension and approximately the same configuration.
Surface light source devices improved as above may be applied to back-lighting-type and front-lighting-type liquid crystal displays for illuminating their liquid crystal display panels from the back side or front side (viewing side). In those cases, the liquid crystal displays reflect characteristics of the surface light source devices. Consequently, a liquid crystal display in accordance with the present invention provides a bright display screen as viewed from a certain direction or position.